Radio frequency Focused Interdigital (RFI) Linac (Technical)

The RFI linac structure is an interdigital linac structure with RF quadrupole focusing incorporated into each drift tube. In interdigital linacs, the electric fields between the drift tubes alternate in direction along the axis of the linac. It is common to support the drift tubes alternately from the top and bottom (or left and right side) of the cavity, as shown in Fig. 1, to support this field pattern.

The longitudinal dimensions of the structure are such that the particles travel from the center of one gap to the center of the next gap in one half of the RF cycle. Hence, particles that are accelerated in one gap will be accelerated in the next gap because, by the time the particles arrive there, the fields have changed from decelerating fields into accelerating fields.

RF focusing is introduced into the RFI linac structure by configuring the drift tubes as two independent pieces operating at different electrical potentials as determined by the RF fields of the linac structure. Each piece (or electrode) of the RFI drift tube supports two fingers pointed inwards towards the opposite end of the drift tube forming a four-finger geometry that produces an RF quadrupole field along the axis of the linac for focusing the beam.

There are three very attractive features of the RFI linac structure: the RF efficiency of the structure is very high, the diameter of the structure is relatively small, and RF electric focusing offers superior performance in the initial portion of the linac.

The RF efficiency advantage of the RFI linac structure over the competition is shown in Fig. 2. The interdigital structure is 4 to 5 times more efficient than the Alvarez structure and 10 to 20 times more efficient than the RFQ in the energy range from 1 to 5 MeV. The RF electric focusing in the RFI linac structure results in better low energy performance and smaller diameter beams throughout the structure, which further enhances the efficiency over magnetically focused structures. The size advantage of the RFI linac structure over the conventional drift tube linac is shown in Fig. 3. The interdigital structure is approximately one third the diameter of the Alvarez structure for the same frequency.

When the accelerated particles are two thirds of the way between the accelerating actions of the RFI structure (i.e. in the latter portion of the drift tube), the electric fields are passing through zero strength and are not suitable for focusing the beam. As a result, the focusing action must be pushed forward (upstream) to lie as close to the accelerating gap as possible, leaving the latter portion of the drift tube solely as a drift action (no focusing, no acceleration). Hence, the drift tubes of the RFI linac structure are asymmetrical, consisting of a minor piece and a major piece, supported on a minor stem and a major stem as shown in Fig. 4.

Fig. 4

At the RF phases of 0° and 180° degrees, the RF electric fields in the gaps between drift tubes are at their maximum strength in a direction to accelerate the beam in every other gap (the accelerating gaps) and in a direction to decelerate the beam in the other gaps. The action of the RFI linac on the beam occurs in three phases, namely the acceleration phase, the focusing phase, and the drifting phase, as shown in Fig. 5.

The acceleration phase occurs at RF phases of -30° and 150° when the RF electric fields are relatively strong and increasing toward their peak magnitude. During this phase, the beam bunches passing through the accelerating gaps are accelerated.

The focusing phase occurs 60° later in the RF cycle, at the RF phases of 30 and 210, when the RF electric fields are still relatively strong and decreasing toward zero. During this phase, the beam bunches that were accelerated in the acceleration phase are now focused and defocused by the action of the RF quadrupole lenses.

The drifting phase occurs 60° later in the RF cycle, at the RF phases of 90 and 270, when the RF electric fields are passing through zero strength. During this phase, the beam bunches simply drift, experiencing no acceleration or focusing forces.

Beam Dynamics

The beam dynamics performance of the RFI linac structure was investigated with the aid of TRACE-3D, a well-known, linear beam dynamics computer program. The effects of the RF acceleration and focusing fields in the RFI linac structure on low intensity beams of charged particles passing through the structure have been analyzed. These calculations establish the capabilities of the RFI linac structure for acceleration of low intensity beams of protons, deuterons, and heavier ions.

At higher intensities, the repulsive electric forces between the charged particle of the beam have a defocusing effect on the beam, tending to reduce the net focusing action provided by the RFI acceleration and focusing fields. The beam current at which this effect jeopardizes the useful performance of the linac structure is referred to as the "space charge limit". The most restrictive space charge limit occurs at the very beginning of the linac where the beam energy is the lowest. The space charge limit of the RFI linac structure was investigated, using the TRACE-3D program, for all combinations of two operating frequencies (100 and 200 MHz) and three injection energies (0.5, 1.0, and 2.0 MeV). In all of these cases, the space charge limits were in excess of 60 mA. At 200 MHz, the space charge limits were in excess of 100 mA. These calculations establish the capabilities of the RFI linac structure for acceleration of high intensity beams of protons, deuterons, and heavier ions.

A PARMILA-like beam dynamics code, PARMIR (Phase And Radial Motion In RFDs), was written to facilitate the study of the beam dynamics in the RFD linac structure. Some modifications were made to this program to support beam dynamics studies of the RFI linac structure. PARMIR now simulates multi-particle beam dynamics in drift tube and interdigital linacs that employ RF focusing inside the drift tubes.

Diagnostic measurements performed on completed RFI linacs successfully confirmed the important aspects of these calculations.